Postsynaptic membrane fragments, containing up to 40 percent of their protein as acetylcholine receptor (AChR), can be prepared from the electric tissue of Torpedo californica. The specific binding and ion-permeability control properties characteristic of AChR function at intact cholinergic synapses are retained in the isolated membrane fragments. In this project the electron paramagnetic resonance technique of spin labeling will be used to characterize molecular motions and lipid-protein interactions of Torpedo AChR in the following three systems: 1) native postsynaptic membrane fragments, 2) membranes specifically modified by phospholipase A2 treatment and 3) reconstituted membrane vesicles prepared from solubilized AChR and characterized lipids. Spin labeled phospholipids, spin labeled alpha-neurotoxin and spin labeled AChR will be used to probe ligand-induced alterations in AChR structure and function. Phospholipase A2 causes a reversible uncoupling of ligand binding and ion permeability control. Characterization of the dynamic lipid and protein interactions in the native and modified membranes will provide useful guidelines for the rational design of the reconstitution experiments. A long-range goal is to provide a complete molecular description of AChR function, including the binding process, the coupling of binding to ion permeability and the ion permeation mechanism itself.